A common industrial method for production of hydrofluoric acid (HF) involves reacting fluorspar ore (containing CaF.sub.2) with sulfuric acid in an externally above, the reaction being: EQU CaF.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2HF+CaSO.sub.4
Minor constituents in the fluorspar also react to give such by-products as SO.sub.2, SiF.sub.4, CO.sub.2, H.sub.2 O, and various phosphate compounds. These materials are present, along with CaF.sub.2 and/or CaSO.sub.4 dust as well as air which may leak into the furnace, as contaminants in the hot HF gas stream leaving the furnace.
Various methods for recovering and purifying the HF are detailed in the patent literature, see for example, U.S. Pat. Nos. 3,919,399; 4,150,102; 4,460,551. Typically, these processes entail steps such as indirect gas cooling, direct gas cooling and purification by contact with sulfuric acid or HF, and product HF recovery by indirect condensation using brine and/or refrigerant. The gases leaving the final HF condensers, containing mainly air, CO.sub.2, SiF.sub.4, SO.sub.2 and other low-boiling compounds, typically contain significant quantities of HF which must be recovered to achieve acceptable process yields.
A standard method for recovering the HF involves absorbing the HF from the gas stream using sulfuric acid, and then using this sulfuric acid as feed acid to the furnace; in this way, the HF is recovered and recycled to the process via the furnace. Additional HF may also be recovered and recycled to the furnace from several of the direct or indirect cooling steps occurring prior to the final product condensation. However, this method of recovering HF is detrimental to furnace operation. Additional heat must be provided to the furnace to vaporize this recycled HF, and 10-20% of the total heat supplied to the furnace may be consumed in this manner. In addition to lowering energy efficiency, this increases the required size of the furnace.
Since the furnace and its auxiliary equipment usually represents the single most expensive item in the cost of constructing an HF plant, the presence of recycled HF in the sulfuric acid fed to the furnace can lead to a significantly higher plant investment cost. The increase in furnace size required to deal with the recycled HF may in fact be greater than the 10-20% expected based solely on heat requirements. The reason for this is that an HF furnace is, among other things, a heat transfer device, and its size is thusly determined not only by heat load but also by heat transfer coefficient. It is desireable that the material in contact with the hot furnace wall have a dry or crumbly consistency. U.S. Pat. No. 4,460,551 teaches that the presence of HF in the feed sulfuric acid (specifically in cold acid, under 100.degree. C.) results in the formation of fluosulfonic acid (HFSO.sub.3) in the feed acid via the reaction: EQU HF+H.sub.2 SO.sub.4 .fwdarw.HFSO.sub.3 +H.sub.2 O
The presence of fluosulfonic acid can lead to formation in the furnace of various intermediate compounds (e.g. calcium fluosulfonate) which produce viscous or sticky materials which cake on the furnace wall, thus reducing the heat transfer coefficient and further increasing the required furnace size. In addition, formation of sticky material is detrimental to the good mixing which is desireable in the furnace, and this can lead to increased residence time requirements, further increasing furnace size.
Several possible methods for reducing HF in furnace feed acid are discussed in the patent literature.
U.S. Pat. No. 3,919,399 presents a method whereby the sulfuric acid prior to being fed to the furnace is contacted with the hot gas leaving the furnace, thus heating the sulfuric acid. However, the claimed maximum sulfuric acid temperature of 120.degree. C. is not high enough to result in a significantly reduced HF solubility, and hence this acid would still contain a high concentration of HF. The effect of temperature on the equilibrium HF concentration of sulfuric acid mixtures is demonstrated in FIG. 2. It can be seen that the equilibrium HF concentration at 120.degree. C. is about twice that at 160.degree. C., which is the preferred operating temperature of the present invention, as described below. A second embodiment referred to in U.S. Pat. No. 3,919,399 mentions "preheating" the furnace feed acid stream to 110.degree.-180.degree.; while at the latter temperature significant HF removal would occur, exotic and expensive materials and construction would be required for such a preheater. Furthermore, in the example detailed in U.S. Pat. No. 3,919,399, the furnace feed acid is stated as containing 4% water; as described below in the discussion of the present invention, this is substantially less than the optimum water content required for HF removal, yet is higher than desireable as regards furnace operation (water, like HF, requires additional heat in the furnace for vaporization).
U.S. Pat. No. 4,150,102 is a refinement of U.S. Pat. No. 3,919,399, the primary objectives of which are improved product quality and reduced equipment pluggage. No mention is made as to the amount of reduction in feed acid HF to be expected, nor is this claimed as a benefit of the patent. The patent detail does state that some HF is evolved from the feed acid; however, as in the case of U.S. Pat. No. 3,919,399, the specified temperature of 120.degree. C. is too low for effective HF removal.
U.S. Pat. No. 4,460,551 details a process for removing HF from the sulfuric acid. This is done in two steps. First, the acid is contacted with the hot furnace gas and heated to 80.degree. C.; in a second step, the acid is heated to 160.degree. C., liberating the majority of the HF. A further improvement in U.S. Pat. No. 4,460,551 is that the major portion of the sulfuric acid is bypassed around the vent gas scrubber and thus does not pick up any HF; the HF-containing sulfuric acid stream, after the bulk of the HF is stripped out, is then diluted with the bypassed acid before being fed to the furnace, resulting in a still lower HF concentration in the furnace feed acid. In order to obtain the desired high furnace feed acid temperature (&gt;100.degree. C.), the bypassed sulfuric acid is heated to 140.degree. C. One practical drawback to the concept of U.S. Pat. No. 4,460,551 is that in order to heat the HF-containing acid to 160.degree. C. and the bypassed sulfuric acid to 140.degree. C., expensive materials of construction must be used, and even then this equipment will be subject to periodic failure, necessitating costly replacement. Furthermore, as will be shown below in the discussion of the present invention, water content is critical in attaining a high degree of HF removal; no means exists in U.S. Pat. No. 4,460,551 for adjusting or assuring an optimal water concentration in the HF removal step, and in fact the stated water concentration of 9% is somewhat less than optimal.
It is thus apparent from the foregoing that a need exists for an improved more efficient and economical process for HF manufacture. There is also a significant incentive in terms of HF furnace operating and capital cost to reduce the level of HF in the furnace feed acid. The present invention provides an improved method for reducing HF in the furnace feed acid, taking advantage of some unusual physico-chemical equilibrium properties of this system. The method of the invention overcomes the drawbacks of the above prior processes and represents a simple, straighforward method for minimizing the HF content of the furnace feed acid by taking advantage of hitherto unrecognized physical and chemical properties of this system.